But since they were named one of the top workout trends in December by the American Council on Exercise (ACE), which does a yearly survey of fitness experts and personal trainers, you have likely heard more about this conditioning method and, perhaps, even seen them in weight rooms and gyms.

A kettlebell is often described as looking like a cannonball with a thick suitcase handle attached. Often made of cast iron, kettlebells were said to first be used by the ancient Greeks. But they have been more recently popularized by their use in Russia and Eastern-block countries to develop whole-body fitness and core strength in athletes.

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The United States has only caught on to kettlebells in the past decade or so, and 2009 marked their first appearance on the ACE fitness trends annual list, suggesting a growing interest in them.

But are kettlebells simply another fitness fad destined to have their brief moment in the limelight before they get retired to a home closet or sit unused in a weight room?

Working Out with Kettlebells

These days, kettlebells might be the trendy thing in the weight room, said Dave Knight, an athletic performance coordinator in the department of sports medicine at the University of Wisconsin Hospitals and Clinics in Madison. But while he has noticed that this Russian training method is finding its way into more fitness centers and is being used more by performance coaches, he said, "We don't use them here [at our fitness facility] because we have other modalities to help athletes develop power."

Although Knight called kettlebells a valuable help with strength training and cardiovascular conditioning, he'd prefer that people work with dumbbells instead.

Unlike dumbbells, which have a bar that distributes two weights equally, kettlebells have a thick handle above a single ball-shaped weight. The higher handle allows a person to swing the kettlebell more freely than you would a similar weight dumbbell.

Kettlebells: A Safe Way to Build Strength?

And while dumbbell workouts typically feature more lifting of the free weight, Knight said, the kettlebell training might find you learning how to swing the weight between your legs, or out in front of you, or from right to left, or on a diagonal. "Swinging of a weight is considered a ballistic exercise, traditionally deemed unsafe," he said.

Of course, you can still do all the usual lifts and moves you would with a weight, such as squats, snatches and dead lifts, with a kettlebell.

And kettlebells come in different weights and often have a color-coded rubber coating so you can quickly tell how heavy or light they are.

As Knight put it, kettlebells are a tool to use for strength training, if they are heavy enough. They can improve core stability, if your posture while using them is good enough, or can be used for power training, if you swing them safely. They even offer aerobic conditioning, if they are light enough so that you do frequent repetitions.

"They may be part of a program, but not the end-all-be-all," he said.

Enthusiasts Describe the Benefits of Kettlebells

Other fitness experts note that kettlebells have some unique features, from their design to the way they are used, that make them worth including in an exercise or injury rehabilitation program.

Kettlebells teach you to use full-body movement from your head to your toes, and they teach people to be much more powerful, said Tim Brewster, a physical therapist and the owner of Train Boston in Wellesley, Mass., a fitness training facility that uses kettlebells for individualized sessions and in group classes.

"They are one of the best ways to teach proper lifting technique to individuals trying to recover from chronic backs injured due to weakness and improper lifting," Brewster said.

And there's another good reason to give them a try, he added: "They're fun."

"We're starting to recognize that kettlebells are a form of lifting that involve range of motions that other forms of exercise don't allow," said Mandla Nkosi, a certified instructor and owner of Boston Kettlebells in Brookline, Mass.

According to Nkosi, the fitness industry makes it easier for people to strength train because machines stabilize the weight for you. Kettlebells, on the other hand, are an awkward tool to lift because the center of mass is displaced from the handle.

Yet it's the very awkwardness of hoisting a kettlebell that Nkosi believes is a good thing.

With most everything you lift -- whether it's the groceries, a child, or snow on a shovel -- the object is never in the optimum place for leverage, he explained. So training with a kettlebell mimics the same kind of lifting you do in everyday life.

For Kettlebells, Proper Positioning Is Key

Whether you're pressing the kettlebell over your head or swinging it forward, you have to learn how to position yourself correctly and manipulate the load. And this, said Nkoski, involves a greater range of motion than conventional forms of strength training.

He said another advantage is that people who use kettlebells stay more supple and loose, whereas people who do other forms of weight training might develop bodies that are really strong, but also really tight and not so good at movement.

And kettlebells differ from other kinds of strength training in yet another key way: Because they involve doing numerous repetitions of exercises where you are swinging a weight, there's lots of cardio work, Nkorski said. More cardio, of course, often means more calories and fat burned during the activity, so "endurance people love it."

Should You Be Trying This at Home?

Despite the many benefits, not all fitness facilities are jumping on the kettlebell bandwagon or singing their praises.

At the University of Wisconsin Hospital's athletic training facilities, Knight preferred dumbbells to kettlebells because, he said, they are less expensive, take up less space, and people could do a lot of the same -- or more – exercises with them.

Carl Davison, the wellness and fitness director at the March Wellness and Fitness Center at the Oregon Health & Sciences University in Portland, is not sold on kettlebells, either, and does not have them in his fitness center.

Davison, a strength and conditioning coach who has worked with professional athletes, as well as the general public, said his center doesn't have kettlebells because of the risk involved.

While he admitted that kettlebells can be a good method of training to increase strength, endurance and flexibility, he also acknowledged that they need to be used safely and with supervision.

Kettlebells a Useful Training Tool for Some

Davison said that the marketing side of fitness has adapted kettlebells to everyday fitness needs and watered them down to the point where they are mostly used in the same way as a dumbbell.

But there's a whole other side to kettlebells that features hard-core devotees and clubs who seriously train and compete with them at a level comparable to competitive weightlifting.

Also, Davison said, ultimate fighters and some firefighters might use kettlebells as a training tool.

Celebrities such as Jennifer Lopez, Penelope Cruz, Cameron Diaz, Sylvester Stallone, Matthew McConaughey and Lance Armstrong are all said to use kettlebells in their exercise programs. And when the stars tout them as a body-sculpting, fat-burning, time-saving fitness training tool, it makes others want to give them a try, too.

Even so, typical gym-goers might not have personal trainers at their side to coach them through workouts or watch their every move like the stars do.